The proposed research is directed at determining the developmental potential of the early omental rudiment of the mouse embryo to form lymphocytes. Preliminary studies indicate that the omental anlage of the early mouse embryo contains stem cells that can form lymphocytes, and it is the purpose of the proposed research work to characterize the omentum-derived cells and to determine both their developmental origin and their subsequent role in immune functions. Specific experimental aims include: 1) Analysis of lymphoid cells formed by the 13-day mouse omental rudiment in transplants or in vitro (differentiation markers, lectin-binding properties, organ-specific antigens; functional studies in vivo and in vitro; colony forming potential); 2) study of histogenesis and morphogenesis of the omental anlage from day 10 of mouse development to birth, including the in vivo and in vitro characterization of stem cell populations; and 3) investigation of the development of omental rudiments with genetic impairment of the hematopoietic (W series), splanchnic mesodermal (Dh/+,Dh/Dh), thymic (nu/nu) or B-cell maturation (CBA/N xid) systems. The basic goal of the proposed research is to study a new source of embryonic lymphocytes in an effort to gain greater understanding of the ontogeny of lymphoid systems in general, and in the hope that the analysis of omentum-derived lymphocytes may define a specific and possibly new subset of cells important in fetal and/or adult immune functions.